1. Field of the Invention
The present invention relates to a sigma delta modulator, and particularly relates to a sigma delta modulator which utilizes a DAC capable of changing configuration.
2. Description of the Prior Art
In the field of signal processing, a sigma delta modulator is often used for processing a signal. Sigma delta modulators can be classified into many types: continuous time, discrete time, one order or multi orders, single bit or multi bits. Normally, if it is desired to increase the dynamic range of an input signal, the number of order or bit of the sigma delta modulator should increase correspondingly. However, the state will be unstable if the number of order keeps increasing, and a non-linear feedback signal is introduced if the number of bit goes high and no extra processing is performed, such that the summed signal becomes non-linear, and results in decreased system linearity. FIG. 1A illustrates the linearity guaranteed single bit DAC and the non linearity of a prior art multi bit DAC. As shown in FIG. 1A, a feedback signal generated from a single bit DAC must be linear, but a multi bit DAC tends to generate a non-linear feedback signal. In this situation, extra algorithms or circuits are needed to maintain or rectify linearity, and as such, additional burden of the system may be needed.
Some sigma delta modulators will change the configuration of the devices to adjust their gain.
FIG. 1B illustrates a prior art sigma delta modulator 100, which is disclosed in Kathleen Philips, etc., “A Continuous-Time ΣΔADC With Increased Immunity to Interferers,” 2004 IEEE Journal of Solid-State Circuits, vol. 39, no. 12, pp 2170-2177. As shown in FIG. 1B, the sigma delta modulator 100 comprises a plurality of variable resistors 101 for adjusting gain of the sigma delta modulator 100. The variable resistor 101 is on the input signal path, which is shown as the variable resistor module 103 in FIG. 1B and the resistance value thereof is based on switch on-off status and is signal dependent. Therefore, the non-constant switch resistance will cause the signal to become non-linear, and thereby result in system distortion. The detailed description of other devices is already disclosed in the paper mentioned above, and thus is omitted for brevity.